Photopolymerizable materials for data storage: from photochemical investigation to customization

Abstract

Different types of polymers are proposed for holographic data storage : photopolymers like PMMA where bonds form or break in the polymer network under illumination, photochromic polymers containing for example azobenzene groups (proposed for high resolution nanolithography), and photopolymerizable systems using inhomogeneous polymerization of one or more monomers for holographic data storage. The material proposed in this work enters in the last family, giving rise to thick phase holograms. The coupling betwween polymerization and diffusion processes is extensively studied in order to characterize the photoinduced microstructuration. Diffusion processes are generated by the concentration gradients due to a disappearance of dye and monomer molecules at different rates in the reactive medium. Creation of gratings with spatial frequencies ranging from 10 to 4000 lines/mm was studied. The formulations are suited to be photopolymerized by illumination around 500 nm, allowing the polymerization of thick samples (thickness of a few hundred microns) with a good optical quality. In order to obtain a reversible process and to improve the storage capicity of the matrix, the medium is doped by a photochromic molecule while the polymerization is used for the photostructuration of the host matrix. The process needs at first the creation of tubular regions corresponding to the highest refractive index of the matrix. By entering in such a fiber, light is guided in the thickness of the material. In each microfiber, bits are recorded in the second stage one after the other one. Several bits can be stored in a same fiber by wavelength multiplexing.